The present disclosure relates to an image sensor.
An image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor may be roughly classified into a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
The CMOS image sensor includes a photodiode region converting a received optical signal into an electrical signal, and a transistor region processing the electrical signal, wherein the photodiode region and the transistor region are disposed horizontally.
In such a horizontal image sensor, the photodiode region and the transistor region are disposed horizontally on a semiconductor substrate. Therefore, the horizontal image sensor has a limitation in expanding an optical sensing region in a limited area, which is generally referred to as a fill factor.
As an alternative to overcome this limitation, an attempt of forming a photodiode using amorphous silicon (Si), or forming a circuitry in a silicon (Si) substrate using a method such as wafer-to-wafer bonding, and forming a photodiode on and/or over the circuitry has been made (referred to as a three-dimensional (3D) image sensor). The photodiode is connected with the circuitry through a metal interconnection.
However, a bonding strength between wafers may be reduced due to uneven boding surface of the wafer. That is, since the metal interconnections between the photodiode and circuitry are exposed on the surface of an interlayer dielectric, the interlayer may have an uneven surface profile. This results in the reduction of the bonding strength with the photodiode formed on the interlayer dielectric.